eGPU Vega 56 (Blackmagic) Mac + cycles problem
You won't be able to use GPU rendering with Cycles. Apple has deprecated OpenCL for macOS 10.14 and future releases. Since macOS won't have any updates for the OpenCL compiler and therefore no bug fixes, Blender 2.80 will no longer use OpenCL on macOS. This was originally announced on the bf-committers mailing list by Sergey Sharybin. It is also documented in the manual and the release notes of Blender 2.80.

AMD OpenCL
OpenCL is supported for GPU rendering with AMD graphics cards. Blender Supports graphics cards with GCN generation 2 and above. To make sure your GPU is supported, see the list of GCN generations with the GCN generation and supported graphics cards.
AMD OpenCL GPU rendering is supported on Windows and Linux, but not on macOS.

A third-party render engine that supports the Metal API may work, for instance AMD Radeon™ ProRender.